This invention concerns an airfoil and more particularly a turbine airfoil whose unique internal construction improves the effectiveness of air cooling at the trailing edge region of the airfoil.
Airfoils constructed with cavities forming passageways for directing cooling fluid therethrough are well known in the art. For example, it is common practice to construct airfoils with spanwise cavities formed within the wider forward portion of the airfoil. These cavities sometimes have inserts disposed therein which define compartments and the like within the cavities. The cooling fluid is brought into the cavities and compartments and some of the fluid is often ejected therefrom via holes in the walls of the airfoil to film cool the external surface of the airfoil.
The trailing edge region of airfoils is generally more difficult to cool than other portions of the airfoil because the cooling air is hot when it arrives at the trailing edge, since it has been used to cool other portions of the airfoil, and the relative thinness of the trailing edge region limits the rate at which cooling fluid can be passed through that region.
A common technique for cooling the trailing edge region is to pass cooling fluid from the larger cavity in the forward portion of the airfoil through the trailing edge region of the airfoil via a plurality of small diameter drilled passageways. Such an airfoil construction is shown in U.S. Pat. No. 4,183,716.
Another common technique for convectively cooling the trailing edge region is by forming a narrow slot between the walls in the trailing edge region, and having the slot communicate with a cavity in the forward portion of the airfoil and with outlet means along the trailing edge of the airfoil. The slot carries the cooling fluid from the cavity to the outlets in the trailing edge. An array of pedestals extending across the slot from the pressure to the suction side wall are typically incorporated to create turbulence in the cooling air flow as it passes through the slot, and to increase the convective cooling surface of the airfoil. The rate of heat transfer is thereby increased, and the rate of cooling fluid flow required to be passed through the trailing edge region may be reduced. U.S. Pat. Nos. 3,628,885 3,819,295, 3,934,322; 3,994,622 4,297,077 and 4,407,632 disclose examples of airfoils constructed in this manner.
Another airfoil constructed with improved means for carrying cooling fluid from a cavity in the forward portion of the airfoil through the trailing edge region and out the trailing edge of the airfoil is shown in U.S. Pat. No. 4,203,706. In that patent, wavy criss-crossing grooves in opposing side walls of the trailing edge region of an airfoil provide tortuous paths for the cooling fluid through the trailing edge region and thereby improve heat transfer rates.
In U.S. Pat. No. 4,437,810 there is disclosed another airfoil having apertures in its trailing edge for cooling air ejection. A metering insert extends between the opposed internal faces of the airfoil adjacent the trailing edge to define the required flow areas.
U.S. Pat. No. 3,864,058 teaches an airfoil having a separate machinable insert within the airfoil and having two separate supply ports for the cooling fluid. One port is in communication with the cooling passage for the suction surface, and the other port communicates with the passages for cooling the pressure surface. The cooling fluid streams are combined and discharged through a slot on the pressure surface of the trailing edge of the airfoil.
The discharge of spent cooling air as a film through high coverage slots on the pressure side wall of the trailing edge of an airfoil, sometimes referred to as pressure side bleed, has become desirable for both structural and manufacturing reasons brought about by the exceedingly thin trailing edges of modern turbine airfoils. A disadvantage of this scheme over conventional air outlets in the trailing edge is its decreased cooling effectiveness, primarily due to a lack of metering capability.
Despite the variety of trailing edge region cooling configurations described in the prior art, further improvement is always desirable in order to function with such thin trailing edge airfoils and allow the use of higher operating temperatures, less exotic materials, and reduced cooling air flow rates through the airfoils, as well as to minimize manufacturing costs.